[dpdk-stable] [PATCH v3 3/3] ring: move the atomic load of head above the loop

[Justin He]

> -----Original Message-----
> From: Gavin Hu (Arm Technology China)
> Sent: 2018年9月26日 17:30
> To: Gavin Hu (Arm Technology China) <Gavin.Hu at arm.com>; dev at dpdk.org
> Cc: Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com>; Steve Capper
> <Steve.Capper at arm.com>; Ola Liljedahl <Ola.Liljedahl at arm.com>;
> jerin.jacob at caviumnetworks.com; nd <nd at arm.com>; stable at dpdk.org; Justin
> He <Justin.He at arm.com>
> Subject: RE: [PATCH v3 3/3] ring: move the atomic load of head above the loop
>
> +Justin He for review
>
> > -----Original Message-----
> > From: Gavin Hu <gavin.hu at arm.com>
> > Sent: Monday, September 17, 2018 4:17 PM
> > To: dev at dpdk.org
> > Cc: Gavin Hu (Arm Technology China) <Gavin.Hu at arm.com>; Honnappa
> > Nagarahalli <Honnappa.Nagarahalli at arm.com>; Steve Capper
> > <Steve.Capper at arm.com>; Ola Liljedahl <Ola.Liljedahl at arm.com>;
> > jerin.jacob at caviumnetworks.com; nd <nd at arm.com>; stable at dpdk.org
> > Subject: [PATCH v3 3/3] ring: move the atomic load of head above the loop
> >
> > In __rte_ring_move_prod_head, move the __atomic_load_n up and out of
> > the do {} while loop as upon failure the old_head will be updated, another
> > load is costly and not necessary.
> >
> > This helps a little on the latency,about 1~5%.
> >
> >  Test result with the patch(two cores):
> >  SP/SC bulk enq/dequeue (size: 8): 5.64
> >  MP/MC bulk enq/dequeue (size: 8): 9.58
> >  SP/SC bulk enq/dequeue (size: 32): 1.98  MP/MC bulk enq/dequeue (size:
> > 32): 2.30
> >
> > Fixes: 39368ebfc6 ("ring: introduce C11 memory model barrier option")
> > Cc: stable at dpdk.org
> >
> > Signed-off-by: Gavin Hu <gavin.hu at arm.com>
> > Reviewed-by: Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com>
> > Reviewed-by: Steve Capper <steve.capper at arm.com>
> > Reviewed-by: Ola Liljedahl <Ola.Liljedahl at arm.com>
> > ---
> >  lib/librte_ring/rte_ring_c11_mem.h | 10 ++++------
> >  1 file changed, 4 insertions(+), 6 deletions(-)
> >
> > diff --git a/lib/librte_ring/rte_ring_c11_mem.h
> > b/lib/librte_ring/rte_ring_c11_mem.h
> > index 0eae3b3..95cc508 100644
> > --- a/lib/librte_ring/rte_ring_c11_mem.h
> > +++ b/lib/librte_ring/rte_ring_c11_mem.h
> > @@ -61,13 +61,11 @@ __rte_ring_move_prod_head(struct rte_ring *r,
> > unsigned int is_sp,
> >  unsigned int max = n;
> >  int success;
> >
> > +*old_head = __atomic_load_n(&r->prod.head,
> > __ATOMIC_ACQUIRE);
> >  do {
> >  /* Reset n to the initial burst count */
> >  n = max;
> >
> > -*old_head = __atomic_load_n(&r->prod.head,
> > -__ATOMIC_ACQUIRE);
> > -
> >  /* load-acquire synchronize with store-release of ht->tail
> >   * in update_tail.
> >   */
> > @@ -93,6 +91,7 @@ __rte_ring_move_prod_head(struct rte_ring *r,
> > unsigned int is_sp,
> >  if (is_sp)
> >  r->prod.head = *new_head, success = 1;
> >  else
> > +/* on failure, *old_head is updated */
> >  success = __atomic_compare_exchange_n(&r-
> > >prod.head,
> >  old_head, *new_head,
> >  0, __ATOMIC_ACQUIRE,
> > @@ -134,13 +133,11 @@ __rte_ring_move_cons_head(struct rte_ring *r, int
> > is_sc,
> >  int success;
> >
> >  /* move cons.head atomically */
> > +*old_head = __atomic_load_n(&r->cons.head,
> > __ATOMIC_ACQUIRE);
> >  do {
> >  /* Restore n as it may change every loop */
> >  n = max;
> >
> > -*old_head = __atomic_load_n(&r->cons.head,
> > -__ATOMIC_ACQUIRE);
> > -
> >  /* this load-acquire synchronize with store-release of ht->tail
> >   * in update_tail.
> >   */
> > @@ -165,6 +162,7 @@ __rte_ring_move_cons_head(struct rte_ring *r, int
> > is_sc,
> >  if (is_sc)
> >  r->cons.head = *new_head, success = 1;
> >  else
> > +/* on failure, *old_head will be updated */
> >  success = __atomic_compare_exchange_n(&r-
> > >cons.head,
> >  old_head,
> > *new_head,
> >  0,
> > __ATOMIC_ACQUIRE,
> > --
> > 2.7.4
Reviewed-by: Jia He <justin.he at arm.com>

Cheers,
Justin (Jia He)
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